1. Field of the Invention
The invention relates to a device for measuring dimensions of containers. More particularly, the invention relates to a container finish measuring device for measuring thread dimensions and ovality of a container finish to determine whether the finish is within a predetermined tolerance limit which will result in a liquid tight seal between the container finish and cap.
2. Brief Description of the Prior Art
In the manufacture of containers, certain dimensions are required to be within predetermined tolerance limits in order for the containers to function properly. In particular, the upper surface of the container, commonly known as the finish, must be maintained within certain manufacturing tolerances in order to provide adequate cap retention for a liquid tight seal to be formed between the container finish and the container cap.
Variations in the configuration of containers, particularly plastic containers, may occur during molding or trimming operations due to many factors, including differences in the molds used to form the containers, shrinkage of the containers after molding, materials used, curing temperatures, and trimming operations. One particular source of warping or ovalization of a container finish is the trimming operation. The container "finish" includes at least the neck, threads, rim, and top surface of a container. In particular, once the bottle or container has been molded, a trimming machine is used to trim flash from the finish of the container. While the flash is being trimmed from the finish, the bottle is held in place by a clamp received around and engaging the finish. Because the container is still warm during trimming, distortion or warping of the finish may be caused by the clamping force used to hold the container.
In order to determine whether containers which are being produced are within predetermined dimensional tolerances, generally a sampling of the containers being produced is measured to determine actual dimensions and ovality. Ovality is the difference between the minimum and maximum diameters of the finish. For containers which will hold fluids, including consumer products such as detergents and bleaches, it is important that the finish of the container be within the predetermined dimensional tolerances so that a cap will be retained properly on the finish and leaks will be prevented.
A bottle finish having threads for retaining a cap includes a T-dimension and an E-dimension. The T-dimension of the finish is the outside major diameter of the threads. The E-dimension is the minor outside wall diameter of the finish at the base of the threads. Generally, a bottle is measured by using calipers to determine the T-dimension of the finish. The T-dimension is measured at a first location along the mold parting line of the bottle, and also, at a second location along a line which is 90 degrees to the mold parting line. These two diameters of the finish are then used to determine the ovality of the bottle finish by subtracting the smaller dimension from the larger. However, these measurements will not give a true ovality of the finish because the two dimensions measured may not be the true minimum and maximum dimensions of the finish.
In order to determine a more accurate ovality, it would be desirable to take a large number of measurements at different positions around the diameter of the finish so that true minimum and maximum dimensions can be determined for calculation of ovality. However, taking a large number of dimensional readings with a caliper to determine an accurate ovality is time intensive.
In addition, the dimensions of a container finish as determined by manual measurement with a caliper will include inaccuracies which vary depending on the expertise of a particular user. The inaccuracies occur due to the manual placement of the caliper, variations in the manual force applied to the caliper, and the caliper blades cutting into the soft material of the bottle finish, such as when the bottle is made of a blown thermoplastic.
As an alternative to manual measurement with a caliper, container finishes may also be measured by an optical comparator. The optical comparator takes an enlarged shadowgraph of the bottle finish to provide a highly accurate measurement of the finish diameter. However, optical comparators are quite expensive and are generally not available at the location where the bottles are made. Therefore, when using an optical comparator, bottles often must be shipped off to a laboratory for measurement, providing a very delayed determination of dimensional tolerances. As a result, a large number of reject bottles may be made before the error is corrected by adjustments to the blow molding and/or trimming processes.
In addition, the optical comparator measures only one dimension of the finish at a time. Thus, as with the manual measurement of the finish with calipers, the bottle must be rotated and remeasured at many locations to determine ovality.
In view of the above drawbacks of the known methods for measuring thread dimensions and ovality of a container finish, it would be desirable to provide a measuring device for accurately measuring the true ovality of a container finish rapidly and with minimal user error.
One type of measuring device which has been proposed for measuring dimensional tolerances of a can includes a base into which the can is placed and the can is then rotated to measure a container dimension. However, this type of device into which a can is placed and then rotated cannot be properly used for a container having a non-symmetrical shape. For example, if a container having an offset finish is set in a device of this type, the container cannot be balanced for rotation within the device. An example of such a measuring device is described in U.S. Pat. No. 5,313,714.
Accordingly, it would be desirable to provide a portable container measuring device which does not require that the container be balanced in the device for measurement. It would also be desirable to provide a measuring device which measures the true maximum and minimum dimensions of a container finish.